Certainly taken aback about dog - it’s a guard-dog? (8)
I believe the answer is:
security
'a guard-dog?' is the definition.
(a guard-dog might provide security)
'certainly taken aback about dog it's' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'taken aback' says the letters should be written backwards.
'about' is an insertion indicator.
'dog' becomes 'cur' (both can mean a despicable person).
'yes' backwards is 'sey'.
'cur'+'it'='curit'
'sey' going around 'curit' is 'SECURITY'.
(Other definitions for security that I've seen before include "Protection from possibility of future financial difficulty" , "State of being free from danger or injury" , "Something given to guarantee payment" , "peace of mind" , "protection or surety" .)